As background, conventionally isoprene has been recovered from pyrolysis gasoline in naphtha cracking to produce ethylene, by a series of fairly capital intensive steps. Initially cyclopentadiene is removed from the pyrolysis gasoline by dimerization and distillation. Subsequently, pipirylenes are separated out by superfractionation, and the isoprene (at 10 to 20% of the pyrolysis gasoline) is then recovered by extractive distillation using a solvent. In recent years, however, with the availability of abundant, inexpensive natural gas, ethylene has increasingly been produced using lighter feedstocks for the steam crackers so that pyrolysis gasoline production has declined.
A number of other petroleum processing-based or -dependent routes to isoprene have been developed as well. As summarized in WO 2012/038247 A1 by Vermeiren et al., “Production of Isoprene from Iso-Butanol” (“WO '247”), isoprene can be produced by the isolation of isoamylenes from refinery and petroleum cuts and the subsequent hydrogenation of the isoamylenes over an iron oxide catalyst promoted with potassium compounds. Isoprene can also be produced from isopentane by a double dehydrogenation. In another known process, isoprene is produced by a Prins condensation of a C4 olefin feed with an aldehyde, typically formaldehyde, with the C4 olefin feed commonly including isobutene or one or more isobutene precursors such as an alkyl-t-butyl ether under conditions which will provide isobutene. The isobutene reacts with formaldehyde to give 4,4-dimethyl-m-dioxane which decomposes to provide the desired isoprene product.
WO '247 reports a number of examples of processes of this general character. For example, U.S. Pat. No. 4,511,751 describes a process wherein isobutene and/or tertiary butanol and a formaldehyde source are fed, together with water, into an acidic aqueous solution continuously or intermittently while maintaining the reaction pressure in an adequate range and at the same time distilling off the isoprene product and unreacted starting materials, together with water, from the reaction zone. U.S. Pat. No. 4,593,145 is cited for describing a process for producing isoprene, characterized in that an alkyl-t-butyl ether (e.g., methyl-t-butyl ether (MTBE) or ethyl-t-butyl ether (ETBE) as are still commercially manufactured and extensively used as antiknock fuel additives) and a formaldehyde source are fed, together with water, into an acidic aqueous solution continuously or intermittently while maintaining the reaction pressure in an adequate range and at the same time distilling off the product isoprene, unreacted starting materials, isobutene and tertiary butanol, together with water, from the reaction zone. Other cited examples include EP 106323, EP 1614671, EP 2157072, GB 1370899 and U.S. Pat. No. 3,972,955.
It is also known to produce isoprene from tertiary alkyl ethers (such as MTBE) and an oxygen source by a catalytic process. For example, in U.S. Pat. No. 3,574,780, isoprene is produced by passing a mixture of MTBE and air over a mixed oxide catalyst, cracking the MTBE to isobutene and methanol, oxidizing the methanol to formaldehyde and then reacting the isobutene and formaldehyde to produce isoprene. Other references supply isobutene and methanol separately. The methanol is oxidized to formaldehyde alongside methanol generated from the cracking of MTBE, and the formaldehyde so formed reacts with the supplied isobutene plus that isobutene generated from the cracking of MTBE. Still other references supply isobutene and methanol directly rather than generating the same by cracking MTBE, oxidizing the methanol to formaldehyde with an oxygen source in the presence of an oxidation catalyst and then reacting the formaldehyde thus formed with the isobutene feed.
While there are thus a number of known methods for producing isoprene, each of these methods has shared the disadvantage of being dependent at least to some extent on petroleum-based or -derived materials and on petroleum processing economics.
In recognition of the desirability of a renewable source-derived isoprene, the WO'247 application provides a process for making isoprene using isobutanol, especially isobutanol produced using biomass as a primary feedstock, as an isobutene precursor. More particularly, at least 25 mole percent of the carbon implicated in the isobutanol is obtained from renewable resources, whether by the base-catalyzed Guerbet condensation of methanol with ethanol and/or propanol, by hydrogenation of synthesis gas from the gasification of biomass and/or by an amino acid biosynthetic route from carbohydrates from biomass. The at least partly bioderived isobutanol in turn provides at least 10 up to 100 percent of the isobutene, with the balance being provided by t-butanol, another isobutene precursor such as MTBE or from fresh isobutene.